Definition of swing
When we talk about swing in poker, we talk about the ups and downs in terms of buy-ins. There are swings up and swings down. When we win a lot of buy-ins, we talk about an upswing. When it will be the opposite, we will talk about downswing. A downswing is caused by the variance in poker (so are upswings). When a negative variance hits us and we lose multiple buy-ins, we'll call it a downswing. A downswing can also be caused by tilt or fatigue. It is not always easy to determine the exact causes of downswing.
Bad luck or bad play?
Sometimes losses are caused by bad luck, but sometimes losses are also caused by tilt. It's a downward spiral. You lose a little bit because of bad luck, it affects you emotionally, you gamble less, you lose even more and you fall into downswing. Conversely, an upswing can make you think you're the best player in the world. When everything works out for us, even when we play badly, we'll talk about upswing. You can't judge the quality of your game by relying solely on the swings you've suffered.